Metal-Catalyzed C–C Bond Cleavage in Alkanes: Effects of Methyl Substitution on Transition-State Structures and Stability

Abstract

Methyl substituents at C-C bonds influence hydrogenolysis rates and selectivities of acyclic and cyclic C2-C8 alkanes on Ir, Rh, Ru, and Pt catalysts. C-C cleavage transition states form via equilibrated dehydrogenation steps that replace several C-H bonds with C-metal bonds, desorb H atoms (H*) from saturated surfaces, and form λ H2(g) molecules. Activation enthalpies (ΔH(‡)) and entropies (ΔS(‡)) and λ values for (3)C-(x)C cleavage are larger than for (2)C-(2)C or (2)C-(1)C bonds, irrespective of the composition of metal clusters or the cyclic/acyclic structure of the reactants. (3)C-(x)C bonds cleave through α,β,γ- or α,β,γ,δ-bound transition states, as indicated by the agreement between measured activation entropies and those estimated for such structures using statistical mechanics. In contrast, less substituted C-C bonds involve α,β-bound species with each C atom bound to several surface atoms. These α,β configurations weaken C-C bonds through back-donation to antibonding orbitals, but such configurations cannot form with (3)C atoms, which have one C-H bond and thus can form only one C-M bond. (3)C-(x)C cleavage involves attachment of other C atoms, which requires endothermic C-H activation and H* desorption steps that lead to larger ΔH(‡) values but also larger ΔS(‡) values (by forming more H2(g)) than for (2)C-(2)C and (2)C-(1)C bonds, irrespective of alkane size (C2-C8) or cyclic/acyclic structure. These data and their mechanistic interpretation indicate that low temperatures and high H2 pressures favor cleavage of less substituted C-C bonds and form more highly branched products from cyclic and acyclic alkanes. Such interpretations and catalytic consequences of substitution seem also relevant to C-X cleavage (X = S, N, O) in desulfurization, denitrogenation, and deoxygenation reactions.

Catalytic Ring Opening of Cycloalkanes on Ir Clusters: Alkyl Substitution Effects on the Structure and Stability of C–C Bond Cleavage Transition States

“…These λ values are larger (by 0.5−1.1) for 3 C− x C bonds (λ = 1.6−4.1) than for 2 C− 2 C and 2 C− 1 C bonds (λ = 1.5−2.9) for each cycloalkane, indicating that the combined number of H atoms removed from the cycloalkane (y, eqs 8 and 10) and desorbed from the surface to bind C atoms (γ, eqs 8 and 10) depends on the degree of substitution of C atoms at the cleaved C−C bond, irrespective of its endocyclic or exocyclic location. These results are similar to the trends in λ values found for 3 C− x C and 2 C− 2 C or 2 C− 1 C bonds in n-alkanes and isoalkanes 18 and are consistent with previously reported effects of H 2 pressure on MCH ringopening rates and selectivities. 8 The mechanistic significance of these λ values for cyclohexanes is readily shown to reflect the sum of the number of C−H (y) and M−H (γ) bonds cleaved to form the relevant transition state (eq 10) because dehydrogenation and adsorption are fully equilibrated (η H → 1, Figure 1) under all reaction conditions.…”

“…These trends resemble the lower C−C rupture rates for 3 C− x C bonds than for 2 C− 2 C bonds as also found for the hydrogenolysis of acyclic isoalkanes. 18 These effects of methyl substituents (Figure 4) are also consistent with the reported preference for ring opening at C−C bonds containing less highly substituted C atoms in five-membered 1,5,6 and six-membered 1,3,4,8,28 alkylsubstituted cycloalkanes. These differences in rates reflect, in part, the larger λ values (by 0.5−1) for 3 C− x C bond cleavage than for 2 C− 2 C or 2 C− 1 C bond cleavage within a given cycloalkane reactant.…”

“…These larger ΔH ⧧ values for 3 C− x C bonds show that the enthalpy of the additional H-removal steps is not fully compensated for by the exothermic formation of the new C−M bonds. 18 These mechanistic interpretations account for λ and ΔH ⧧ values that are larger for the hydrogenolysis of C−C bonds at the most substituted positions in both alkyl cyclohexanes (Table 3) and acyclic isoalkanes, 18 and such seemingly general trends indicate that these concepts also account for different values for ΔH ⧧ and rate constants for C− C cleavage at substituted and unsubstituted positions in other cycloalkane reactants (e.g., cyclopentanes and polycyclic species) and on other catalytic surfaces (e.g., Ru, 1,18 Rh, 18,27,49 Ni, 1 and Pt 1,6,18,27 ).…”

“…17 Experiments and transition-state theory treatments, based on statistical mechanics, show how and why C−C bond cleavage rates differ between 3 C− x C bonds and 2 C− 2 C and 2 C− 1 C bonds within a given isoalkane on Ir, Rh, Ru, and Pt clusters. 18 Briefly, 3 C− x C bond cleavage in acyclic alkanes is mediated by transition states that involve the removal of 4−5 H atoms from the isoalkane reactants and the concomitant formation of one C−M bond (where M represents an exposed metal atom) for each C−H bond ruptured. 3 C atoms can form only one C− M bond, thus requiring the additional dehydrogenation of C atoms not involved in the 3 C− x C bond cleaved.…”

“…This sequence of endothermic steps increases ΔH ⧧ values and forms gaseous H 2 molecules, leading to ΔH ⧧ values that are larger for 3 C− x C bonds (by 20−70 kJ mol −1 ) than for 2 C− 2 C and 2 C− 1 C bonds. 18 These larger ΔH ⧧ values are compensated for, in part, by the significant entropy gains arising from the H 2 (g) molecules formed; the formation of 3−4.5 H 2 molecules for each transition state contributes 400−500 J mol −1 K −1 to these ΔS ⧧ values. As a result, such activation entropies become large and positive (160−300 J mol −1 K −1 ), in spite of the entropy losses associated with the binding of the reactant alkanes.…”